Airbag supplemental restraint systems (SRS) have become increasingly pervasive because of their ability to effectively protect vehicle occupants from serious injury in the event of a collision. The typical airbag system has an airbag, an inflation device, and a crash sensor that detects a sudden deceleration of the vehicle. In order to be effective, airbags are deployed rapidly at speeds of over 200 mph and are, therefore, prone to cause injury to small children or to passengers who are improperly restrained. To prevent such injuries, vehicle occupant sensors are employed to help determine whether or not the airbag should be deployed during a collision.
A number of techniques have been used to engineer vehicle occupant sensors. For example, a strain gauge can be placed in a car seat in order to determine whether there is a weight on the seat. A disadvantage of using a strain gauge, however, is that that the airbag may needlessly deploy if the seat is occupied by objects and packages, such as groceries, instead of a passenger.
Some vehicle occupant sensors work by detecting a change in an electric or magnetic field in the presence of a passenger by using, for example, a capacitance sensor or an inductive coil placed under the seat. Such systems, however, typically operate at a single frequency and are unable to determine the position and size of the seat occupants.
What are needed are circuits and systems that can more accurately detect the presence of a passenger in a vehicle car seat.